Subject: [Sigtran] Applicability of M3UA AS-PENDING state to ASPs and IPSPs

From: "Daniel Cohn" <[email-masked]>
Date: Thu, 8 Sep 2005 10:29:31 -0500

I have a question regarding the queuing of outbound messages while in
the AS-PENDING state.  RFC 3332 section 4.3.2 contains the following
text:

   AS-PENDING: An active ASP has transitioned to ASP-INACTIVE or ASP
DOWN
   and it was the last remaining active ASP in the AS.  A recovery timer
   T(r) SHOULD be started and all incoming signalling messages SHOULD be
   queued by the SGP. If an ASP becomes ASP-ACTIVE before T(r) expires,
   the AS is moved to the AS-ACTIVE state and all the queued messages
   will be sent to the ASP.

Is this meant to imply that queuing is only performed by the SGP and
never by the ASP or IPSP?  If so, what does the ASP/IPSP do while
waiting for the recovery timer to expire?  Does it simply throw away all
outgoing messages?  This would seem to defeat the purpose of
implementing the AS-PENDING state in an ASP or IPSP.

Thanks,
Dan

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From: "Brian F. G. Bidulock" <[email-masked]>
Date: Thu, 8 Sep 2005 11:35:25 -0600

Daniel,

A couple of comments:

- T(r) can be zero (0), and there is no recommended value, so the
  entire AS-PENDING state is optional.

- An IPSP (SGPF) can enter AS-PENDING state.

- With regard to ASPs, see the third paragraph of section 4.3.4.5:

  "In the case where a Notify message ("AS-PENDING") message is sent by
   an SGP that now has no ASPs active to service the traffic, or where a
   Notify ("Insufficient ASP resources active in AS") message is sent in
   the Loadshare or Broadcast mode, the Notify message does not
   explicitly compel the ASP(s) receiving the message to become active.
   The ASPs remain in control of what (and when) traffic action is
   taken."

  Also note that the normal action for MTP-Users is to receive
  notification (via one of the methods indicated in Q.701) of isolation
  of the user part from the SS7 network.  No user part queues messages
  after such an indication.

- Using an AS-PENDING state at the SGP can have a negative impact on the
  SS7 network (delaying detection of failed signalling points and
  unavailable user parts) and User Parts (e.g. causing ISUP calls to
  experience excessive post dial delay rather than being sent to
  reorder).

- As there is no equivalent in SS7 (MTP never intentionally queues
  messages for any period of time to known unavailable user parts or
  known failed signalling point codes), IMO the value of zero (0) should
  be used for T(r) for all failure situations when connecting to public
  networks.

- AS-PENDING is an SGP-specific state (not an SG-wide state).  Even
  though a single SGP is in AS-PENDING state does not mean that messages
  cannot be eventually routed to the AS via another SGP in the SG, or
  event via the SS7 network (e.g. C-links).  In such a case, T(r) would
  be akin to a time-controlled rerouting timer; however, SS7 uses forced
  rerouting in such situations (immediate), and T(r) equal to zero (0)
  is still advisable in those situations.

- If you can't set it to zero, I would set it to the minimum possible
  value, hopefully milliseconds and not seconds.

- If you want to avoid missequencing and duplication of messages (for
  high performance or public networks), see the corrid draft.

- About the only use for AS-PENDING is in an Override AS to smooth the
  transition from a failed ASP to the standby, when it is known that the
  standby is ASP-INACTIVE and otherwise healthy (i.e. it is capable of
  taking over the traffic from the failed ASP.)

-- brian

Daniel Cohn wrote:                                                                  (Thu, 08 Sep 2005 10:29:31)

-- 
Brian F. G. Bidulock
[email-masked]
http://www.openss7.org/